1. Field of the Invention
The present invention relates to a cylinder block structure for an internal combustion engine and, more particularly, to a cylinder block structure for an internal combustion engine having a main body of a cylinder block with cylinder bores and a lower cylinder block section to be fixed to a bottom end of the main body of the cylinder block.
2. Description of Related Art
U.S. Pat. No. 4,831,978 discloses a general structure of a conventional cylinder block wherein a skirt section serving as a space for accommodating a crank shaft is so arranged as to extend to an outwardly downward extent at its lower portion and wherein an oil pan is mounted to a bottom end of the skirt section. More specifically, a rear end portion of the skirt section of the cylinder block is in such a shape that its outer side portion is extended more and more in an outwardly rearward fashion as it approaches its rear end, and a transmission is mounted to a rear end of the skirt section thereof.
Further, Japanese Patent Laid-open (kokai) Publication No. 61,757/1988 discloses a cylinder block having a plate member so interposed between the skirt section thereof and the oil pan as to suppress the skirt section thereof from contracting or expanding.
On the other hand, Japanese Patent Laid-open (kokai) Publication No. 108,612/1987 discloses a cylinder block wherein a side wall of the skirt section is bolted to a side portion of a bearing cap supporting a crank shaft in order to enhance rigidity for supporting the crank shaft. Further, this prior publication discloses a plurality of bearing caps whose bottom ends are connected to each other with a beam constituted by a separate member.
It is to be noted herein that the interposition of the plate member between the skirt sections, the fastening of the bearing cap to the skirt section, and the disposition of the beak for connecting the bearing caps to each other are intended to decrease the vibration of the internal combustion engine and, as a consequence, lead to a decrease in a "power plant bending phenomenon". The term "power plant bending phenomenon" and related terms are intended herein to mean a phenomenon wherein a power plant containing the internal combustion engine and the transmission is caused to bend and transform in a curved shape as the internal combustion engine vibrates. This so-called power plant bending phenomenon is known as inducing vibration of the vehicle body.
Recently, there is one of the growing demands for internal combustion engines for automotive vehicles that the total weight of the engine should be made lightweight. In order to meet such a demand, attempts have been made of the structure of a cylinder block in which the lengthwise dimension of the skirt section, as called "short skirt", is shortened and a lower cylinder block section of the cylinder block is coupled to the bottom end of the shortened shirt section while an oil pan is mounted to the bottom end of the lower cylinder block section. For such a cylinder block of a so-called short skirt type, for example, the main body of the cylinder block is made of cast iron and the lower cylinder block section is made of aluminium, thereby making the total weight of the cylinder block lightweight.
Further, for the cylinder block of a so-called short skirt type, the lower cylinder block section is considered as a member for supporting the transmission and, in this respect, attempts has so far been made exclusively in order to ensure rigidity for the power plant bending phenomenon.